1. Field of the Invention
This invention relates to a movable contact member of a slide switch, and more particularly to a slide switch having a structure suitable for preventing undesirable heating of the components of the switch.
2. Description of the Prior Art
A slide switch such as an air control switch is composed of an insulator, at least one conductive stationary contact member fixed in an upper surface of the insulator and defining a boundary between it and the insulator, a contact holder freely slidable on the stationary contact member, and a movable contact member supported on the slidable contact holder through a contact biasing spring and having at least one conductive rivet fixed to it to make sliding electrical contact with the stationary contact member.
When, at the boundary between the insulator and the stationary contact member, the rivet engages the insulator at one of halves of its contact surface and engages the stationary contact member at the other half of its contact surface in a very narrow range, the associated contact surface of the stationary contact member and that of the rivet fixed to the movable contact member is oxidized. In other words, when the contact area between the stationary contact member and the rivet fixed to the movable contact member is very small, the sectional area of the path of electrical current is decreased to increase the electrical resistance, thereby heating the stationary and movable contact members. Also, when the rivet provided on the movable contact member is slightly displaced from the stationary contact member, a spark jumps across the corresponding portions of them, and heat attributable to the spark (Joule heat) is generated. Due to generation of heat from these portions, the functions of the other parts of the switch will be adversely affected by the heat, or the useful service life of the movable or stationary contact member will be shortened.
That is, when the distance of sliding movement of the movable contact member, after the slide switch is turned on, is shortened due to oxidization of the contact surface of the rivet or due to a shortened on-off stroke of the slide switch, the contact resistance increases to increase the electrical resistance, and Joule heat resulting from a voltage drop is generated to heat the movable and stationary contact members. As a result, the contact holder and the insulator may be damaged by fusion or burning.
Undesirable generation of the Joule heat can be prevented by slightly shifting the position of one of the movable and stationary contact members relative to the other. However, in the case of the slide switch which is turned on and off under control of a servo motor, the on-off positions of the movable contact member relative to the position of the stationary contact member are automatically determined. Therefore, even when the electrical contact between the movable and stationary contact members at each of intermediate positions may be ideal, the position where the movable contact member makes electrical contact with the stationary contact member cannot be accurately predicted.
Jp-A-50-107469 describes an electrical contact mechanism of a slide switch in which a stationary contact member is formed of a bimetal so that the contact area of the contact member increases with a temperature rise. However, this slide switch is defective in that, when the switch is used in an environment where the ambient temperature is considerably low, the area of electrical contact between the stationary contact member and its mating movable contact member is very small, resulting in the possibility of generation of heat at the contact area.